Looking calm in an interview with a local television network, Graca Machel did not give details about Mandela's health status, just saying it was painful to see the nonagenarian "aging."
"I mean, this spirit and this sparkle, you see that somehow it's fading," she told ENews Central Africa (ENCA) on Monday in her first interview since Mandela was hospitalised at the weekend.
South African government officials have said the former President is comfortable and does not face immediate danger, but they refuse to speculate on when he is likely to be discharged from a Pretoria military hospital.
Mandela, 94, was at the weekend admitted to hospital for tests that authorities say are expected of people of his age.
"To see him aging, it's something also which pains you. . . . You understand and you know it has to happen," said Graca.
Mandela's granddaughter Ndileka told the same TV network that he has taken to accept his condition.
"I think he takes it in his strides, he has come to accept that it's part of growing old, and it's part of humanity as such. At some point you will dependent on someone else, he has come to embrace it," she said.
Defence Minister Nosiviwe Mapisa-Nqakula visited Mandela on Monday and said the revered statesman was "doing very, very well".
The presidency said it was too early to give an update as they have to hear first from the doctors.